With the recent progress of technological development, there are increasingly strict and complicated requirements for materials in terms of strength, function, safety, cost, etc. Since materials consisting of one material can only meet such requirements in a limited manner, composite materials comprising two or more kinds of materials are being increasingly used. Among them, composite materials comprising a joint of metal and resin are in frequent use. Examples of these combined materials include: electric wires comprising metal wires covered with a resin film; circuit boards for use in electronic equipment comprising integrated metal and resin; and automobile/airplane parts comprising metal and metal, or metal and resin, which are bonded with an adhesive.
Following such technological development, Japanese Laid-Open Patent Publication No. 2001-316872 proposes a composite material comprising a firm joint of metal and resin which utilizes both insert molding and bonding. However, while the technique of making metal-resin composites progresses, the establishment of recycling technologies is strongly desired in view of environmental problems regarding global warming and waste disposal. The recycling technologies have also been promoted by the force of law since the Electric Appliance Recycling Law went into effect.
As in the slogan “If mixed, it's waste; if separated, it's a resource”, the basics of recycling are to separate materials by kind. Products comprising various constituent materials can be recycled only when they are decomposed/separated into their respective constituent materials. Home electric appliances, in particular, include a large number of components comprising a metal-resin joint. Therefore, there is a strong demand for a simple method of separating these components into their respective constituent materials.
The decomposition/separation methods of composite materials are roughly classified into three kinds: mechanical methods; chemical methods; and electrochemical methods. In the case of an electric wire, for example, a mechanical method in which the electric wire is cut into a suitable length and the resin film is separated from the metal wire has been devised. Separating resin from metal by a mechanical method is relatively easy in such cases as the electric wire in which the metal portion and the resin portion are not bonded with an adhesive or the like. However, if the metal portion and the resin portion are firmly bonded, it is basically impossible to separate the two portions by a mechanical method.
Therefore, conventionally in use is a method in which a composite material comprising a metal-resin joint is incinerated and valuable metals are collected from the remaining ashes. This method, however, requires a large amount of energy and presents a problem of carbon dioxide and dioxin emissions.
There is also a well-known method in which a composite material comprising a metal-resin joint is frozen in liquid nitrogen or the like to embrittle the resin and is then impacted to destroy the resin portion in order to recover the metal portion (U.S. Pat. No. 4,406,411, U.S. Pat. No. 4,043,019, and U.S. Pat. No. 4,020,992). However, this method requires high cost for separation because of the necessity to use liquid nitrogen or the like and, in addition, is not capable of completely separating the resin from the metal in principle.
Another known method of separating a metal-resin joint shreds a composite material by a shredder. The particulate material obtained by shredding is subjected to centrifugation (U.S. Pat. No. 5,948,276), or is dispersed in liquids having various specific gravities (U.S. Pat. No. 5,616,641), in order to separate the composite material into its respective constituent materials. It is difficult, however, to completely separate resin from metal by shredding using a shredder.
Also, Japanese Laid-Open Patent Publication No. Hei 6-166769 proposes a method in which a composite material of metal and resin is heated and thereafter pressed by twin rollers in order to separate the resin from the metal portion. However, this method requires a large amount of energy for heating and pressurization and involves destruction of the metal portion, although the separation of the resin from the metal is possible.
On the other hand, Japanese Laid-Open Patent Publication No. Hei 8-327512 proposes an electrochemical method of separating the resin film from a plated steel plate coated with the resin film. This method immerses a plated steel plate coated with a resin film in a methanol solution containing iodine ions and maleic anhydride and anodically dissolves the plated layer applied on the surface of the steel plate. However, this method separates the resin film from the steel plate by maintaining the steel plate at such a potential that the plated layer such as zinc is selectively dissolved, so the method is applicable only to limited metals such as the zinc-plated steel plate and therefore lacks versatility.
Further, Japanese Laid-Open Patent Publication No. Hei 9-187751 proposes a method of removing copper foil or solder from a printed circuit board made of phenol resin by immersing the printed circuit board into a hot alkaline solution and then into water. However, although this method is effective when the resin portion of a composite material is phenol resin that easily swells with alkaline solution, it is not suitable for other composite materials.
As described above, each of these conventional methods of separating a metal-resin joint has its own drawbacks and generally involves destruction of the article comprising a metal-resin joint.